In motor vehicles, refrigerant circuits are outfitted with a heat pump function in order to make the heat from the surroundings or the waste heat from the vehicle usable for heating of the interior compartment of the vehicle at low outdoor temperatures. The refrigerant compressors in this case work in temperature ranges that substantially exceed the level of the usual vehicle air conditioning, especially at low outdoor temperatures.
DC link capacitors of various types can be used in inverters of the refrigerant compressors. Due to their high capacitance density, low price, ready availability, and ability to operate at high temperatures, DC link electrolytic capacitors have become widespread for use in electrical refrigerant compressors in motor vehicles.
The problem with electrolytic capacitors and especially for those types with high working voltage is that the capacitance decreases and the impedance increases greatly with dropping temperature. The effect even occurs exponentially at temperatures below −25° C. This confines the use of electrolytic capacitors to temperature ranges above −20° C.
But electrical refrigerant compressors in motor vehicles are also operated at temperatures below −20° C., especially when they are also designed for heat pump use, in order to utilize the ambient air in heating mode for the heating of the vehicle.
Alternatively, film capacitors or ceramic capacitors can also be used, but these have specific disadvantages. Film and especially ceramic capacitors have a substantially lower capacitance density as compared to electrolytic capacitors and are much more costly and larger than comparable electrolytic capacitors for high voltages and high capacitance values. Furthermore, film capacitors are usually not suitable for high temperatures of over 110° C. The disadvantage with large ceramic capacitors is that they are sensitive to impact and vibration, which can well occur in motor vehicles.
A method and a device for control of an electric motor are known from U.S. Pat. Appl. Pub. No. 2009/0039813 A1. The method for control of the motor voltage for the electric motor is realized by making use of an inverter. The inverter has an inverter circuit and a capacitor, which is connected to the input of the inverter circuit. The method realizes the control of the inverter circuit in a way that regulates the electric voltage for the electric motor in terms of the temperature of the capacitor.
Thus, the problem of the invention is to enable the use of relatively small and economical electrolytic capacitors even for temperature ranges below −20° C.